1. Field of the Invention
The present invention relates to a power supply apparatus. More specifically, the present invention relates to a direct current power supply apparatus employing a direct current/alternate current inverter, particularly suited for small sized and light weight electrical equipement.
2. Description of the Prior Art
A self-excited inverter employing a Royer oscillator has been proposed and put into practical use as an inverter for use in a power supply apparatus. Since such a Royer type inverter is well known, it is not believed necessary to describe the same in detail. Briefly described, a Royer type inverter comprises two power transistors which are each utilized as a switching device such that a direct current is on/off controlled or switched to provide an alternate current voltage of a rectangular wave form, wherein the switching operation of the transistors is achieved by a feedback coil coupled to the primary winding of a saturable transformer. In operation of such a Royer type inverter, one transistor is rendered conductive while the other transistor is rendered non-conductive for a given time period and then the operation state is reversed and thereafter such an operation is repeated. In such a Royer type inverter, the excited voltage in the primary winding of the saturable transformer serving as a common load of the respective transistors causes a voltage as high as approximately two times the power source voltage to be applied between the collector and emitter electrodes in the non-conduction state. Accordingly, transistors of a higher withstand voltage are required for such an inverter.
A so called half bridge type inverter has also been proposed as an improvement on the above described Royer type inverter. A typical example of a half bridge type inverter is shown in FIG. 1. For facility of understanding of the present invention, the structure and operation of the half bridge inverter 1 shown in FIG. 1 will be described to the extent as desired or necessary. When a switch 3 is turned on, a direct current voltage from a direct current voltage source 2 causes a current to flow through base resistors 7 and 11 of a pair of PNP transistors 6 and 10 and a voltage drop across the respective base resistors 7 and 11 is about to render the pair of PNP transistors 6 and 10 conductive. It is seen that a starting resistor 13 is connected between the base and collector electrodes of the transistor 10. For this reason, the current flowing through the base resistor 11 is larger than the current flowing through the other base resistor 7 and hence the transistor 10 is rendered conductive earlier than the transistor 6. When the transistor 10 is rendered conductive, the electric charge in a dividing capacitor 9 is discharged through the transistor 10 and a primary winding 5 wound on a saturable magnetic core 14. On the other hand, a dividing capacitor 4 starts being charged by a current flowing through the transistor 10 and the primary winding 5 from the direct current voltage source 2. At that time, the current flowing from the point a to the point b of the primary winding 5 induces a voltage across one feedback winding 12 in the direction for forward biasing the transistor 10 and also induces a reverse bias voltage for the transistor 6 in the other feedback winding 8. Accordingly, the transistor 10 is rendered conductive, while the transistor 10 is rendered non-conductive, with the result that a positive half wave output is generated in a secondary winding 15 wound on the magnetic core 14 and coupled to the primary winding 5.
Thus, the primary winding 5 is excited. When the primary winding 5 is thus excited and the magnetic core 14 of the saturable transformer is saturated, the magnetomotive force of the magnetic core 14 disappears and a voltage is induced in the respective feedback windings 8 and 12 in the reverse direction. Therefore, one transistor 10 is rendered non-conductive and the other transistor 6 is rendered conductive. Accordingly, the electric charge in the dividing capacitor 4 is discharged through the primary winding 5 and the transistor 6 and the dividing capacitor 9 is charged by a current flowing through the primary winding 5 and the transistor 6 from the direct current voltage source 2. The above described charging current causes the primary winding 5 to be excited in the direction from the point b to the point a, with the result that a negative half wave output is obtained in the secondary winding 15.
The output from the inverter 1 is obtained through the secondary winding 15 wound on the saturable magnetic core 14 and coupled to the primary winding 5 and is applied to a load circuit 200, which may comprise a direct current motor, for example.
Since in such a half bridge type inverter as shown in FIG. 1, the voltage E of the direct current voltage source 2 is divided by two by means of a pair of dividing capacitors 4 and 9, the voltage E is applied between the collector and emitter electrodes of the transistor in conduction. As a result, according to a half bridge type inverter, transistors of a lower withstand voltage can be employed as compared with a case of a Royer type inverter.
However, even a half bridge type inverter still has several problems to be solved. More specifically, one transistor 10 need be provided with a starting resistor 13 for preferentially rendering the other transistor 10 conductive. Because of the starting resistor 13, the bias circuits for two transistors 6 and 10 selected as a pair in terms of the electrical characteristics become asymmetrical. However, this causes a situation that the transistor 10 is not rendered fully non-conductive or cut off, i.e. a somewhat conductive tendency is left even in the non-conduction period. In order to avoid such a situation, it could be considered that a reverse bias is applied between the base and emitter electrodes of the transistor 10, although there remains some difficulty. Another problem is that the above described conductive tendency of the transistor 10 makes the output wave of the inverter 1 asymmetrical with respect to the positive and negative polarities, with the result of poor efficiency. This fact is also liable to cause a short circuited state of the pair of transistors 6 and 10.